Fluid flow control valves of the above-described type are particularly used for regulating fluid flow to a substantially constant flow rate over a range of pressure differentials, such as about 0.1 bar to 10 bars. In such valves, the diaphragm typically comprises a solid body of elastomeric material. When urged against the seat of the orifice, the diaphragm deforms, the degree of deformation increasing with increasing pressure differential across the diaphragm. As the deformation of the diaphragm increases, the flow control passages between the diaphragm and the seat become smaller. The valve is designed such that over the range of pressure differentials of interest, the changing flow area of the flow control passages offsets the changing pressure differential so as to maintain the flow rate substantially constant.
A common type of flow control valve employs a "torpedo" shaped diaphragm that has an outer peripheral surface of smaller diameter than the inner surface of the housing of the valve. In normal forward flow through the valve, fluid flows between the outer peripheral surface of the diaphragm and the inner surface of the housing and then is turned radially inwardly by the orifice and flows through the flow control passages between an end face of the diaphragm and the orifice seat.
Some manufacturers of constant-flow valves have machined various sculptured shapes into the seats of orifices and have assembled each different orifice with a common elastomeric diaphragm identical in shape from one valve to the next. The diaphragm hardness typically was specified as 68.+-.5 Shore A durometer hardness. Because the allowable variation of ten counts of durometer hardness results in significant variations in flow rate for a given orifice, this manufacturing technique lead to a batch production process in which custom-designed orifices were produced for each production batch of elastomeric diaphragms, depending on the hardness of the batch of diaphragms.
The flow rate through the flow control passages between the diaphragm and orifice is proportional to the flow area of the passages multiplied by the square root of the pressure. Accordingly, the flow area of the flow control passages must change significantly from the lowest working pressure to the highest working pressure (e.g., from 0.1 bar to 10 bars) in order to maintain the flow rate substantially constant at all pressures. Various approaches have been taken to try to tailor the deflection of the diaphragm against the orifice seat so as to maintain approximately constant flow rate over the working pressure range. One prior approach employed a plurality of small projections of very small contact area on the orifice surface that engage the diaphragm in an attempt to increase the flow area at the low end of the pressure range. At low pressure differential, as the pressure differential increases the projections press into the diaphragm and locally deform it and the face of the diaphragm moves closer to the main surface of the orifice seat. A drawback of this approach is that the very small contact area of the projections causes a significant hysteresis effect. The valve also tends to have higher than desired flow in the pressure range where the diaphragm deflection makes a transition from local deformation to bending.
Another prior approach was to limit the deflection of the diaphragm to pure compression without bending, as shown in U.S. Pat. No. 3,189,125. This was accomplished by making the diaphragm of sufficient thickness and shaping the diaphragm to have its thickest section at the center so that substantially no bending would occur. This approach works well for differential pressures exceeding 1.0 bar. However, when the operating range is expanded to include pressure differentials below 1.0 bar, problems begin to arise.